Incorrect samplerate from RTL-SDR

Richard Koch n1gp at hotmail.com
Thu Jan 9 22:53:30 UTC 2014


Hi Steve,

In regards to your post about the anti-aliasing filter
you referred to, it appears that it is getting set in
librtlsdr.c, rtlsdr_init_baseband()

If one wanted to change that to optimize the anti-aliasing
to a lower bandwidth, say 1500 Khz instead of the 2048 Khz

How would you change the fir_coeff[] table entries to achieve
this? The 8 bit entries and 12 bit entries are confusing to me.


==== orig post ====

> I have seen other reports on the mailing list, saying that sample rates
> between 300 kS/s and 900 kS/s don't work. If this applies to all RTL
> devices, then maybe it should be documented and the library can return
> an error code for such sample rates. Otherwise people will keep walking
> into this trap.

We could return some sort of error in those cases, but such low rates
(< 1MS/s) aren't recommended in general. First of all, the
anti-aliasing filter we're using has a fixed bandwidth of 2 MHz, and
although the coefficients can be changed, I doubt you can get a nice
filter for lower bandwidths with such a low order. And then the ADCs
only have 8 bits resolution, so you want to improve that by decimating,
and also profit from decimation gain.
Even Realtek uses 2.048 MS/s for FM reception in their original Windows
software. 		 	   		  
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